"He likes to exercise, especially swimming."
Duolingo stated that, as a general rule, they use and require 很 in front of adjectives. You're right, though.
This exercise isn't consistent with the "他很喜歡做運動，特別是遊泳。"/"He likes to exercise, especially swimming." exercise; that exercise uses "做" for the same English sentence, but this one forbids use of "做".
Duolingo stated that they use 很 in front of adjectives and require it for this course as a general rule.
As I mentioned above, 喜欢 is not an adjective. There are lessons in this course where 很 is omitted in front of 喜欢.